Question

\(\mathrm{Al}_{2} \mathrm{O}_{3}\) is amphoteric. What does this mean?

Short answer

Aluminum oxide (Al₂O₃) being amphoteric means it can act as both an acid and a base. As an acid, it donates protons (H⁺) to a base according to the reaction: \[Al_2O_3 + 6OH^- \rightarrow 2Al(OH)_6^-\]. As a base, it accepts protons (H⁺) from an acid following the reaction: \[Al_2O_3 + 6H^+ \rightarrow 2Al^{3+} + 3H_2O\]. This dual behavior allows Al₂O₃ to interact with both acids and bases depending on the solution it is in contact with.

Step-by-step solution

Understand Al₂O₃ as an Acid

Amphoteric compounds can donate protons (H⁺) to a solution, which is a characteristic behavior of acids. In the case of aluminum oxide (Al₂O₃), when it's in contact with a base, it can donate protons to the base. The chemical reaction for aluminum oxide (Al₂O₃) acting as an acid is: \[Al_2O_3 + 6OH^- \rightarrow 2Al(OH)_6^-\] As you can see, aluminum oxide donates protons (H⁺) to hydroxide ions (OH⁻) present in the base, forming hexahydroaluminate ions (Al(OH)₆⁻).

Understand Al₂O₃ as a Base

Amphoteric compounds can also accept protons (H⁺) from a solution, which is typical behavior of bases. In the case of aluminum oxide (Al₂O₃), when it's in contact with an acid, it can accept protons from the acid. The chemical reaction for aluminum oxide (Al₂O₃) acting as a base is: \[Al_2O_3 + 6H^+ \rightarrow 2Al^{3+} + 3H_2O\] As you can see, aluminum oxide accepts protons (H⁺) from the acid, forming aluminum ions (Al³⁺) and water (H₂O).

Synthesis

In summary, aluminum oxide (Al₂O₃) is amphoteric because it can both donate protons (H⁺) to a base and accept protons (H⁺) from an acid. This dual function allows Al₂O₃ to act as either an acid or a base, depending on the solution it's in contact with.